Electrical contact protection network



May 24, 1949. g, MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4Sheets-sheaf. 1

. INVENTOR R. C. MA 7HES ATTORNEY May 24, 1949.

R. c. MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4Sheets-Sheet 2 Mil EN ron R. C. AM THE S ATTORNEY 1949- R. c. MATHES2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4Sheets-Sheet :5

Fl 8 FIG 7 G C M R n 1:

Cl I" IN VE N TOR By RCZMATHES A TTORNE'V y 1949 R. c. MATHES 2,470,825

ELECTRICAL CONTACT PROTECTION NETWORK Filed April 10, 1945 4Sheets-Sheet 4 FIG. [3

lNVE/VTOR R. C. MA THES ATTORNEY Patented May 24, 1949 ELECTRICALCONTACT PROTECTION NETWORK Robert C. Mathes, Maplewood, N. J., assignorto Bell Telephone Laboratories, Incorporated, New York, N. Y., acorporation of New York Application April 10, 1945, Serial No. 587,462

2 Claims.

This invention relates to contact makers and breakers and particularlyto the protection of contacts from the ill eil'ects of starting andstopping the flow of comparatively heavy electrical currents.

An object of this invention is to provide an 7 improved type of circuitpresenting a constant v resistance impedance to the contacts for thepurpose of controlling both the breaking voltage and the closing currentto the least damaging value corresponding to a given type of contact.

A further object of this invention is to provide a circuit arrangementwhich will particularly apply to a pair of contacts connected inparallel and intended to be operated in synchronism.

Contact protection becomes a problem where an attempt is made to providea contact maker and breaker of a size and capacity to economicallyhandle its ordinary duties but which must not be allowed to fail underemergency conditions which are liable to arise in service. For instanceit is common knowledge that the resistance of the ordinary tungstenfilament lamp rises greatly as the filament attains incandescence,whereby the current when the circuit for such apparatus if first closedis much greater than it is a short period thereafter. It will,therefore, be seen that if the circuit is closed and then almostimmediately opened the contacts will be called upon to break a currentmany times heavier than usual. If the contacts through erosion havebecome other than perfect such an opening after a closing is not anuncommon occurrence and the result is further erosion and perhapssticking contacts through welding or mechanical interlocking of theroughened surfaces.

It has been found expedient in certain circumstances in order to insuredependability in service to use a pair of contacts in parallel and whilethis goes along way toward solving the problem it does not provide acomplete answer.

It has been discovered that the requirements for contact protection onthe opening and on the closing of a circuit are mutually conflicting. Byway of example, it is common practice to connect a condenser about apair of contacts to protect them against the ill eflects of breaking aheavy current. Therefore, such a condenser becomes charged during theopen periods of the contacts so that when they are closed the first rushof current therethrough is not only that due to the normal circuitfunctioning but also the discharge current of the condenser. The greaterthe capacity such a condenser has, the greater is the protectionprovided on the opening of the contacts, but unfortunately the greateralso is the initial rush of current when the contacts are broughttogether.

In accordance with the present invention a type of circuit novel to theart of contact protection is provided for insertion between the contactsand the load circuits. This consists of a uniform line or one of thatclass of circuits known as constant resistance networks by means ofwhich the breaking voltage and the closing curient can be held toconstant controlled values for the short times of importance in openingor closing.

In accordance with the present invention a novel means is provided toprotect parallel contacts consisting essentially of contact protectingnetworks for each contact pair arranged to mutually effect each otherwhereby cancellation of heavy transients may be brought about. A feature, then, of the present invention is a circuit network applied toeach of two circuit makers and breakers arranged to operatesimultaneously, connected in such manner that the interlinked magneticcircuits of the two branches of such circuit network are in mutual andopposing relation to each other.

The drawings consist of four sheets having thirteen figures, as follows:

Fig. 1 is a schematic circuit diagram showing the relations of certaintheoretical circuit elements in a contact protection network;

Fig. 2 is a graphical representation of transient voltage phenomena;

Fig. 3 is a similar graphical representation of transient currentphenomena;

Fig. 4 is a graphical representation of voltage and current phenomenawhere a theoretically perfect section of ideal line is inserted betweenthe contacts and the remainder of the circuit;

Fig. 5 is a schematic circuit diagram showing a constant resistancenetwork of the form of Fig. 'l inserted between a pair of contacts andthe load which it is to control;

Fig. 6 is a similar schematic circuit diagram showing a constantresistance network of the form of Fig. 8 inserted between a pair ofcontacts and the load which it is to control;

Fig. 7 is a fundamental schematic circuit diagram of one form ofconstant resistance network;

Fig. 8 is a similar fundamental schematic circuit diagram of anotherform of constant resistance network;

Fig. 9 is a compromise contact protection network which will givepractically as good results as a theoretically good circuit;

Fig. 10 is a still further simplification in a compromise circuit;

Fig. 11 is a schematic circuit diagram showing an arrangement for theprotection of two simultaneously operated contact pairs;

Fig. 12 is the same circuit redrawn to explain the conditions presentwhen one of the contacts opens before the other; and

Fig. 13 is an equivalent circuit useful in explaining the theory ofoperation when the coil is so constructed that through leakage there isless than perfect mutual induction between the parts thereof.

In the study of erosion of relay contacts it has been found thatphenomena at both the opening and the closing of the contacts are ofgreat importance. Work has been done on the design of protectioncircuits designed in general to control the rate at which voltage acrossthe contacts develops in the former case and the rate at which currentcan build up through the contacts in the other case. tion circuit isexpected to take'care of both cases.

In the case of opening the contacts, the ill effects result fromestablishing and sustaining,

Usually a single invariable protecfor periods of a few microseconds to afew milliseconds, a destructive metallic are, a gaseous are or bothtypes alternating. The existence and which mayv result in welding or inthe transfer of material due to certain electrothermal effects. Theseverity of these effects depends chiefly on the contact materials, thebattery voltage and the capacita'tive character of the load.

The palliative'measures required for the two cases are to a degreemutually conflicting and the final design must in general be acompromise with the relative seriousness of the two factors oftenremaining to be determined by experimental means. How this comes aboutcan be readily seen by examining in a step-by-step fashion the buildingup of a typical protection circuit for an inductive load.

In Fig. 1 we have an inductive load which will first be assumed to be apureinductance L with a resistance RL. The load current Ir. will beequal to the battery voltage En, divided by RL. We are interested in thevoltage Eo when the contacts I and 2 are opened and the current when thecontacts are closed, for several simple protection conditions providedby the conventional shunt circuit of Rc-C. If neither is present we knowfrom common knowledge of transient current phenomena that the voltage E0produced at the contacts I and 2 on opening the circuit thereat will bein the form of curve I of Fig. 2 and the current produced therethroughon the closing of these contacts will be in the form of curve 2 of Fig.3. This is a very bad condition for voltage arcing and a very favorablecondition for welding current control. If now we add the capacity C butomit Rc we will get curve 2 of Fig. 2 for Eoand curve I of Fig. 3 forId. The relative severity for the-two conditions is now reversed and inmost cases we would be little better off.

These curves have been oversimplified in order to bring out the generalnature of the circuit reactions. This is particularlytrue of the numberI curves designed to illustrate the ap roach to infinite voltage orcurrent if a nearly perfect opening or closure were obtainable. Actuallythe dotted portions of the curves are extremely irregular in practice.In the case of breaking, different types of transients such as thosedescribed by A. M. Curtis in an article entitled Contact phenomena intelephone switching circuits published in the Bell System TechnicalJournal,

January 1940. P ges 40-82, may be set up, with intermittent low voltagemetallic arcs or high voltage gaseous arcs. Similarly in the case ofclosure, the energy stored in the condenser may be dissipated in"preclosure arcs as well as in the rush of current dissipated as simpleresistance heating.

If now we add the resistance Re into the network of Fig. 1 and satisfythe condition that Rc= RL=.\/CJ- we will get curve 3 in both Figs. 2 and3. E0 is now equal to En for all time. after the contact opening and I0is equal to later all time after contact closing. This comes aboutbecause the impedance looking into the network from the contactterminals is a constant resistance for all frequencies and cannot,therefore, bedistinguished from a single perfect'resistance.

Unfortunately Ea will generally be a higher voltage than can bepermitted ta appe'ar across the contacts atv the momentof openingwithout serious arcing. As a first step in exploring a method forproviding a controlled voltage, let us insert between the contacts andthe restof the circuit of Fig. 1 a section of ideal-line'ofcharacteristic impedance R0 and a time of transmission tx from one endto the other thereof. The rest of the circuit will still be assumed tomeet the constant resistance relation stated'above. With the contactsclosed there will flow the normal closed circuit current of IL. Uponopening the contacts the overall circuit will function as an in finiteline of true resistance impedance until the first reflection arrivesfrom the end of the line. That is, up to a time 2tx, there will be anopen circuit voltage across the contacts.

Eo=ILRo (1) This is shown in Fig. 4 for Ro Ro or RL. At longer times thevoltage reaches the value Ea. Likewise when the contacts are open wehave the line charged to a potential EB and for the same interval oftime after closing we will derive a current through the closed contactsof Ic=Es/Ro (2) Thus by making the impedance of the uniform line Ro lessthan R0 or R1. we have been able to control matters for a short lengthof time so that the open circuit voltage conditions are made morelenient at the expense of making the short circuit condition moresevere. However, the latter is not as severe as curve I of Fig. 3. Inthe case of very high current loads working fromlow voltage battery, thereverse result could have been secured by making R0 greater than Re.Depending on the characteristics of the load and/ or other protectionequipment between the load and the assumed uniform line; the dottedparts of the curves in Fig. 4 may approach the final steady value eitherby slow approach or by E0 and I0 will need to be determinedexperimentally for every physically different pair of contacts at theirnormal operating speeds and pressures. Once these constants have beendetermined we know under ideal conditions what wattage load thesecontacts will handle to some degree independent of whether they areoperated from a high or low voltage battery. The adjustment to differentbattery voltages is made simply by the selection of the characteristicresistance R of the uniform line.

Under practical conditions there are several factors which preventreaching the full emciency of use indicated by the formula (3). 0n thebreak, relay chatter or "reclosures" will require that better protectionthan indicated may be required. The phenomena of reclosure" frequentlyoccurs after the relay contacts have been somewhat roughened by theerosion process. On closure there is another phenomena which developswhen the battery voltage is above 50 to 100 volts. This is known as"preclosure" and is presumed to be due to point discharges initiating anarc due to the high voltage gradients as the contacts come very closetogether and is discussed in the Curtis article hereinbefore noted.

For some limited fields of use small artificial lines, such as may bemade by winding a solenoid to have a high capacity to ground, will beuseful devices according to this invention. However such structures willin general be impractical for a great many situations, to meet which itis desired to provide other forms of constant resistance networks. Twoof these are shown in Figs. and 6 derived respectively from thewell-known forms of constant resistance networks shown in Figs. '7 and8. In Figs. 5 and 6 the impedance looking to the right from the contactswill be a constant resistance R if the impedance looking to the rightfrom points 3 and l is also a constant resistance R and if R'=Li/C1.This can be made approximately true down to some fre quency h if C: andLa are made large enough. Then the constant resistance network properwhich controls the initial voltage or current for an initial period t1corresponding to the uniform control time 2t! of Fig. 4 is that part ofthe network between the contacts and the points 5 and 6.

The product L101 will be larger the longer the time over which it isdesired to hold the initial i i= l The resonant frequency of this meshwould appear'to be a reasonable one to which to maintain the constant Bcharacteristic, so

.and

1 fl "Ti;

At this frequency. 11, the reactance of the condenser C: should be smallcompared with that of the inductance L1. The resonant frequency 1: ofLa-C'n will thusbe considerably lower than {1 and the time constants ofthat mesh much longer. If it is desired to lengthen the time of flatvoltage or current control without going to larger values of L1 and C1(and hence also of La and C2) this can be done by adding additionalsections identical with those included between the contacts and thepoints I and 4. Each of these if properly terminated to the right thenautomatically ro' vides the constant R terminating impedance required bythe section to the left. Other more complicated types of constantresistance circuits may be employed, especially if it is desired tocompensate for parasitic capacitance or inductance.

The next section of the iigures may be looked upon as a low-pass filteror might be designed as another constant resistance section designed toput another step in the curve of transition to the final current orvoltage values. This would have an impedance between that of the firstsection and the final section. It will thus not be possible tomatchthese sections and also have both of them constant resistance. Anapproximation' as shown in Fig. 9, however. may give very satisfactoryresults. Here R1 plusRa would be made equal to Re and R; would be equalto Cs would have its re-' actance small compared with that of Le at thefrequency I:

In many cases encountered in telephone practice the simpler form ofcircuit shown 'in Fig. 10 may sufiice. In the case of a heavy telephonerelay'load the values for Fig. 10 would be about as follows:

I1.==.4 ampere En=50 volts Eo== initially -8 volts whereupon R =-i20ohmsand for ti=.0001 second.

C =%9- -5 microfarads C: should be about ten times C1. If t1 must beheld for one millisecond, the above figures for C1 and Limust bemultiplied by ten or if it must be held to only 10 microseconds they maybe divided by ten. This shows the value of a high initial accelerationof opening if simple contact protection circuits are to be attained.Actually the lower figure would probably meet most telephone cases.

For heavy loads such as small motors with 5 to 10 amperes current thecharacteristic impedance becomes very low and R or R1 will be about 1ohm. For these heavy currents it becomes especially important to holdthe free wiring between the contacts and the protection network to aminimum. Such wiring is a short piece of uniform line of about 500 ohmscharacteristic resistance. Hence, at 5 amperes we would get an initialvoltage of 2,500 volts lasting .002 microsecond per foot of wiring.Condensers should also be free of parasitic inductance.

In cases where the load conditions do not impose too stringentrequirements, the circuit of Fig. 6 reduces to a form of protectionwhich has often been used, simply the first series element and the firstshunt condenser. In this situation the series element has been made bywinding the inductance coil upon a .conductive permeable core. Thisprovides the approximate equivalent of a coil shunted by a resistanceand provides a small unit which can readily be mounted very close to thecontacts to be protected, the importance of which has alread beenindicated.

The design of contact protection circuits is thus seen to be one of manycompromises which should always be thoroughly investigatedexperimentally. The importance or what happens in a i'ew microsecondsdepending on the very high frequency properties of connected circuitelements cannot be over-emphasized. A feature the constant resistancemethod of design is that the open circuit voltage can be held uniformalter the -initial jump for a selected short interval of time(corresponding to these critical first few microseconds) instead oistarting to rise immediately as is the case with the more conventionaltype oi protection circuit.

The principles of this invention may be further applied to the operationof two contacts inparallel. The use of double contacts is becomingincreasingly more common so the use of this arrangement does not becomeunduly expensive in practical use.

The arrangement shown in Fig. 11 is one in which two contact pairs IIand I 2 are intended to simultaneously open the circuits therethroughand to thus control a secondary circuit. When the device is normal aconducting path is closed between the conductors l3 and II and when thedevice is operated this path is opened.

Between the conductors l3 and I4 there is one path containing acondenser lli and another path containing in series a resistance It anda condenser I'I. Between the conductor l3 and the 8 with the conditionmaintained that !z.=Ii+l:. Then the energy stored in the coil ield whichmust be, dissipated in the resistances is proportional to 12-11approachinglr. as a maximum if the difi'erentlal time oi opening islong.

Hence, at the worst, contact I! it it opens at a considerable time aftercontact I I (considerable in relation to the time constant of thiscircuit) will have to bear the full brunt oi the opening to the samedegree thatthe single contact in Fig. 6 does. The more nearly the twocontacts operate at the same time, the more benefit will be derived fromthe circuit.

However, it we look at the situation on closing,

made with considerable leakage between the two The situation here seemsto be in a general way 1 similar to that in the case of a singlecontact; the circuit requirements for opening and closing are mutuallyconflicting and final design will be a matter of compromise. It is ofinterest in this connection that in certain cases the diilerence in timebetween the opening of two contacts may be quency impedance looking intothe circuit from the contact ll (opened as indicated in Fig. 12) whilethe contact I! is still closed, we will get essentially a short circuitwith high mutual inductance between the two halves of the coil l9 and2|. Hence at the moment the circuit is opened at contact ii, if theother is still closed, we would expect a voltage build-up starting fromzero similar to curve 2 of Fig. 2.

If we measure the impedance at the contacts Ii with contacts i2 open wewill get a resistance at high frequencies. Hence, if the two contactsopen together (or at the instant the contacts I 2 open after thecontacts II have opened) we would get a controlled voltage jumpcorresponding to the differential division or the two currents in theupper and lower halves of the coil. If the currents divided equally asthey normally would and the two contacts opened at the same instantthere would be no voltage jump and we would systematically difierentfrom that corresponding to closing' For example, it has been observedthat the difference in time of closing of two glass sealed contactsunits whose magnetic circuits are operated in parallel may be as high asone millisecond. The difference in the time of opening, however, willnot be more than or microseconds. This systematic diiference may be usedto advantage in the design of the coil as it will dictate the amountofmutual impedance in the coil.

What is claimed is:

1. The combination of a pair of circuit makers and breakers connected inparallel and arranged to operate simultaneously, of a constantresistance protecting network in series with each consisting of aninductance shunted by a resistance, said inductances .being mutuallycoupled.

2. The combination of a pair of circuit makers and breakers connected inparallel and arranged to operate simultaneously, or a constantresistance protecting network in series with each consisting of aninductance shunted by a resistance, said inductances being mutuallycoupled and a condenser bridged about said combination.

ROBERT C. MATHES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,929,057 Dellenbaugh Oct. 3,1933 2,285,691 Wegener June 9, 1942 2,394,389 Lord Feb. 5, 1946

